Inks used in the ink-jet printing industry are typically liquid dispersions, solutions, or emulsions. Known types of ink include oil-based inks, non-aqueous solvent based inks, water-based inks, and solid inks. The ink-jet printing process involves jetting droplets of ink from orifices of a print head onto a print medium. Then, the deposited ink droplets are either affirmatively dried, e.g., using heat or forced air, or allowed to dry at ambient conditions.
Recently, curing of ink by radiation, and in particular ultraviolet (UV) curing, has become popular. In these cases, special radiation-curable ink is used and the image is cured by exposure to a radiation source. The term “curing” in the context of the present application refers to a process of converting a liquid, such as ink, into a solid by exposure to actinic radiation. The use of radiation-curable inks and the curing process are rapidly becoming an alternative to the established conventional drying process.
Among the problems associated with the use of many types of inks is poor adhesion to non-porous or low surface tension substrates such as polypropylene, vinyl, polyolefins, and the like. Polypropylene, for example, is widely used for rigid and semi-rigid Point-of-Purchase displays. Because the surface of the polypropylene is inert and has low surface tension, polar solvents such as acrylic monomers will not wet it. As a result a poor adhesion is obtained. Additionally, depending on the ink load, when printing inks on top of one another, proper cohesion between ink layers on substrate can be less than desirable. Further, with specific reference to UV curable inks, these inks are often prepared using oligomers such that the inks have a viscosity that is much higher than more typical solvent or water based inks. Jetting of such viscous inks presents certain difficulties, and further, can typically only be jetted at low frequencies, causing slower printing speeds. Because these inks are more viscous, they are also usually jetted at elevated temperatures, e.g., above about 40° C. This elevated temperature allows some control over the inks otherwise high viscosity, but adversely affects the life of printing architecture including the print head. Thus, there is significant room for improvement in the area of UV curable inks, particularly as it relates to adhesion to polypropylene or other non-porous, low surface tension substrates.